This invention relates to fuel cell systems and, in particular, to the removal of electrolyte from gas oxidizers used in such systems.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Fuel cells operate by passing a reactant fuel gas through the anode, while passing oxidizing gas through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell forming a fuel cell stack.
Molten carbonate fuel cells (“MCFCs”) operate by reacting oxygen in the oxidizing gas and free electrons at the cathode to form carbonate ions, which migrate across the molten carbonate electrolyte to the anode to react with hydrogen and produce water, carbon dioxide and electrical power. The electrolyte in the MCFCs comprises a molten carbonate salt mixture, which usually includes lithium carbonate, potassium carbonate, or a combination of lithium and potassium carbonates. Because the operating temperature of the MCFC is approximately 550-650° Celsius, the electrolyte is in liquid state during MCFC operation.
Typical MCFC systems include an anode exhaust gas oxidizer unit downstream from the fuel cell anode, which comprises an oxidation catalyst for oxidizing hydrogen, carbon monoxide and unreacted hydrocarbons in the anode exhaust to produce oxidizing gas suitable for adding to the air or oxidant gas for supply to the fuel cell cathode. In some cases, the oxidizer unit is incorporated into an exhaust gas oxidizer assembly which includes a mixer which precedes the oxidizer unit. In this assembly, the anode exhaust gas stream and the cathode supply air or oxidant are first mixed in the mixer and then the mixed gases are fed into the oxidizing unit for oxidizing the exhaust gas via exposure to the oxidation catalyst in the unit. The resultant gas which is rich in oxidant and carbon dioxide is then fed to the cathode of the fuel cell.
U.S. patent application Ser. No. 10/187,495, assigned to the same assignee hereof, discloses an example of such an anode exhaust gas oxidizer assembly employing a mixer/eductor and an oxidizer unit in which the gas oxidizer catalyst block of the oxidizer unit interfaces with the outlet of the mixer/eductor. In this oxidizer assembly, the gas mixture of anode exhaust gas and air enters the oxidizer catalyst block through the inlet face of the catalyst block coupled to the outlet of the mixer. In the oxidizer block, the gas mixture undergoes a combustion reaction to oxidize hydrogen, carbon monoxide and hydrocarbons present in the mixture.
During the fuel cell stack operation, some electrolyte particles are in a gas phase and are released from the electrolyte layer of the fuel cell into the anode exhaust stream. After the hot anode exhaust stream is mixed with cool air in the mixer, the temperature of the resulting gas mixture is approximately 300 to 400° Celsius. Due to the cooling of the anode exhaust, the electrolyte particles in gas phase in the exhaust stream are transformed from gaseous particles into solid electrolyte particulates. These particulates are deposited on the walls of the mixer and at the inlet face of the oxidizer catalyst block. The electrolyte particulate deposits on the inlet face of the oxidizer catalyst block create a partial obstruction of the flow path of the gas mixture into and through the oxidizer catalyst, resulting in an increased pressure drop across the catalyst block and thus an increasing difference between the pressure of the anode exhaust stream and the cathode inlet stream. In addition, the blockage by the electrolyte deposits changes the flow distribution through the catalyst block, resulting in a larger difference in temperature distribution from one end of the catalyst block to the other.
The performance and efficiency of a fuel cell stack is sensitive to pressure changes in the fuel cell assembly. Particularly, the increasing pressure difference between the anode and the cathode streams due the aforementioned accumulation of electrolyte particulate deposits on the oxidizer catalyst affects the thermal profile and voltage variations of the fuel cell stack. Moreover, electrolyte particulate deposits deactivate the oxidizer catalyst which affects the hydrocarbon combustion efficiency.
Accordingly, in order to maintain the pressure difference between the anode outlet and the cathode inlet streams constant, it is necessary to remove the electrolyte particulate deposits from the inlet face of the anode gas oxidizer catalyst. In addition, removal of electrolyte particulates reduces the deactivation of the oxidizer catalyst and improves fuel cell performance.
Conventionally, electrolyte particulates have been removed from the anode gas oxidizer catalyst by washing the oxidizer catalyst with a solvent suitable for removal of alkali carbonate compounds. This method, however, requires the fuel cell plant to be shut down and the oxidizer assembly to be disassembled to remove the catalyst block for washing. As a result, the efficiency of the fuel cell power plant declines and maintenance costs of fuel cell power plant operation significantly increase. Therefore, an in-situ method of removal of electrolyte particulates is needed so as to avoid these disadvantages.
It is therefore an object of the present invention to provide for removal of electrolyte particulates from an oxidizer assembly having an oxidizer unit of a fuel cell system which does not require the removal of the oxidizer unit from the oxidizer assembly.
It is a further object of the present invention to provide the aforesaid electrolyte particulate removal in a manner which does not significantly affect the performance of the fuel cell system.